Siberut macaque description

The Siberut macaque (Macaca siberu) is a medium-sized monkey endemic to the Mentawai Islands, and is one of two macaque species that exist there. Although once thought to be the same species, these two macaques are now considered to be separate. The Siberut macaque is darker in colour than its close relative, the Pagai Island or Mentawai macaque (Macaca pagensis) (2)(3), and also shows genetic differences (5).

Like other macaques, the Siberut macaque is a rather heavily built monkey with a very short tail. Male macaques are generally larger than the females (6). The Siberut macaque is mostly black above, with brown on the flanks and arms. Silver cheeks and a pale grey-brown to whitish throat and belly break up its otherwise dark fur. This species has black skin on its hands, feet and face (2)(3), and possesses a very short beard with hairs less than one centimetre in length (2). The Siberut macaque’s tail is haired only near the base, and is hooked forwards at the tip (2)(3).

As well as being darker than the Mentawai macaque, the Siberut macaque is also distinguished by its distinctive pale cheek patches, and it lacks the buff-coloured sides of the neck shown by the other species (2)(3). Infant Siberut macaques are born with whitish fur and with reddish skin on the face, hands and feet. Their fur changes to a more adult-like colouration after a few weeks (7).

Groups of Siberut macaques are known to give a range of different vocalisations. Adult males also produce a loud, harsh bark which can be heard up to a kilometre away and may serve to direct the group in its morning foraging activities (2).

Related species

Siberut macaque biology

Macaques have a reputation for being very sociable animals which often live in large groups (6). The Siberut macaque has a smaller than average group size, but can still be found in groups of around 5 to 25 individuals (9), which may include a number of adult males and females (7). Smaller subgroups are formed for foraging in the day and sleeping at night (2). The Siberut macaque can also form mixed-species groups with the Mentawai langur (Presbytis potenziani) (9).

Siberut macaques have an interesting way of maintaining positive interactions with each other, using a ‘bared teeth’ display to show their peaceful and friendly intentions. This involves opening the mouth and retracting the lips, showing the teeth (12).

The Siberut macaque primarily eats fruit (2)(7), and like the Pagai Island macaque it may potentially raid gardens and coconut groves in search of a meal (10)(11). In addition to fruit, the Siberut macaque also eats some arthropods, including termites, ants and spiders, as well as mushrooms, leaves, shoots and flowers. It has also been reported to catch and eat freshwater crabs and shrimps (2)(7).

This species forages for most of the day and sleeps in tall trees at night, sometimes within its subgroups. The groups do not generally use the same tree twice (2). The Siberut macaque is semi-terrestrial, spending much of its time on the ground (7), and if alarmed it will typically drop from the trees and flee across the forest floor (2). Groups spend a relatively large part of each day travelling in search of food (7). Once a food source is located, the Siberut macaque often eats rapidly, transferring large quantities of food to its cheek pouches to be eaten later (2).

In general, macaques breed seasonally, with mating taking place during a fairly limited period (6). In the Siberut macaque, births have been recorded between March and April, between September and October, and in January and July (7). As in other macaque species, female Siberut macaques are likely to show their receptiveness by displaying their swollen and reddened genitals to the males of the group. After a gestation period of five to six months, macaques give birth to a single infant. The infant clings to the female’s belly as soon as it is born, and develops a close relationship with its mother until adulthood or, if it is a male, until it reaches sexual maturity and leaves the group (6).

Potential predators of the Siberut macaque include eagles and large snakes (2).

Species with a similar range

Siberut macaque habitat

Siberut is mainly covered in tropical lowland rainforest, with canopies of 24 to 36 metres in height, made up predominantly of large, buttressed trees (8).

The island also has a variety of other habitat types, including mangroves, secondary forest, riverine forest, beach forest, palm groves, gardens and logged forest. The Siberut macaque has been found in all of these except gardens and beach forest, but it prefers primary forest as this provides a wide range of fruit for foraging, as well as tall trees for nesting in at night. The macaques may venture into disturbed habitats to forage (2).

Siberut macaque status

Siberut macaque threats

One of the major threats to the Siberut macaque is hunting by humans (2)(7)(13). This species is considered a crop pest by locals, and hunting has become even more of a problem recently with the introduction of rifles to the island, as well as improved access to remote areas through the building of logging roads. The Siberut macaque is also hunted for food, and infants are often taken for the pet trade. Hunting of this species used to be regulated by local taboos, but these have unfortunately been largely abandoned (13).

Another significant threat to the Siberut macaque is commercial logging, which is increasing on the island, greatly reducing this species’ habitat (13)(14). Siberut is less affected by habitat loss than the other Mentawai islands, but has still lost over 50 percent of its forests, and several major logging companies hold concessions to operate there (14). Large tracts of land are also being cleared to make way for oil palm plantations, and the forest is being cleared and products extracted from it by local people (13).

Large trees are particularly important to the Siberut macaque, both as a source of fruit and as sleeping trees. Selective logging which takes out these large forest trees would therefore be detrimental to this species. As the forests on the island are degraded and fruit supplies reduced, the Siberut macaque may also become more likely to raid crops on farms, bringing it into greater conflict with local people (7).

Siberut macaque conservation

The Mentawai islands are part of the Sundaland ‘biodiversity hotspot’ as they contain an unusually high number of endemic species, including four primates, and are therefore of particular conservation interest (5)(15). The Siberut Conservation Programme is currently working to preserve the remaining forests of Siberut and, as a result, the population of Siberut macaques that depends on it (14).

The Siberut macaque occurs in one protected area, the Siberut National Park (1), although this currently needs greater protection (13). This species is listed on Appendix II of the Convention on International Trade in Endangered Species (CITES), meaning that international trade in the Siberut macaque should be carefully controlled (4), but its legal protection in Indonesia is uncertain as it has only recently been recognised as a distinct species (1).

Recommended conservation actions for the Siberut macaque include formal protection for the Peleonan forest in the north of the island, which contains large primate populations. Long-term conservation success on the island will also depend on the involvement of local people, and on changing attitudes towards hunting and sustainability. The development of alternative incomes for local people will be important to reduce the likelihood of their land being sold to logging companies, so helping to protect the habitat of the Siberut macaque (13).

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